Calciners and particularly indirectly-heated calciners are utilized for heat-treating and drying of materials at temperatures higher than those normally employed in steam-heating apparatus. For example, indirectly-heated calciners having a rotating cylindrical retort are frequently employed to reduce mineral oxides to low oxides, dry and remove sulfur from cobalt, copper and nickel powders, reduce metal oxides, and numerous other similar applications wherein relatively high temperatures are desirable.
In the known types of calciners employing a rotating cylindrical retort, it is a common practice to feed the material for processing into one end of the retort and collect the processed material from an opposite discharge end. Also, a series of flights attached to the inner surface of the rotatable retort or a "scraper" chain may be utilized to control the progress of the material as it is processed through the rotating cylindrical retort.
Although the above-mentioned flights and scraper "chains" are necessary to insure continued progress of the materials through the cylindrical retort, a problem exists in that it would be highly desirable to increase the retention time or the time during which the material being processed remains in the cylindrical retort. Such an increase in retention time obviously leads to a highly desirable increase in loading capacity and enhanced utilization of the apparatus.
One known technique for achieving the above-mentioned increase in material retention time is to introduce a dam or screen into the discharge end of the cylindrical retort. Thus, the contained materials must either reach a height sufficient to spill over the dam or be of a size small enough to pass through the screen. Unfortunately, it has been found that both dams and screens are undesirably accompanied by pluggage of the retort. Moreover, attempts to clear the undesired pluggage result in undesired and expensive shutdown time or, in the case of screens, replacement thereof due to perforations encountered in attempts to unplug the screen. In both cases efficiency and costs suffer.